A conventional boiling water reactor includes a reactor core disposed in a reactor pressure vessel, with the reactor pressure vessel being disposed in a drywell vessel which in turn is disposed in a containment vessel. The pressure vessel includes an inlet feedwater line which channels water to the pressure vessel which is heated by the reactor core for generating steam. The steam is discharged from the pressure vessel through an outlet steamline to power a conventional turbine-driven generator for producing electrical power, for example.
Disposed in the containment vessel is a conventional suppression pool which includes a portion also within the drywell vessel with flow through the drywell vessel being provided by a plurality of horizontal vents therein. The suppression pool serves various conventional functions such as, for example, a heat sink and a condenser for any steam released in the drywell area.
In addition to the basic systems which control operation of the reactor, reactor auxiliary systems are provided which are used during normal and abnormal plant operation. Exemplary auxiliary systems include the reactor core isolation cooling (RCIC) system, the residual heat removal (RHR) system, the emergency core cooling system (ECCS), and the automatic depressurization system (ADS).
The RCIC system maintains sufficient water in the reactor pressure vessel to cool the reactor core in the event the vessel becomes isolated from feedwater flow and electrical power. A conventional RCIC system includes a turbine operatively joined to the pressure vessel steamline and a pump driven by the turbine which is operatively connected to the vessel feedwater line. In operation, the steam from the pressure vessel powers the turbine which in turn powers the pump for providing makeup water from a conventional condenstate storage tank or the suppression pool into the feedwater line for maintaining water level in the pressure vessel. The steam discharged from the turbine is channeled to the suppression pool which acts as a heat sink.
The RHR system includes one or more heat exchanges operatively connected to the suppression pool for removing heat therefrom which is added thereto by the RCIC system or by the condensation of steam therein for example.
The ADS is operated in conjunction with the ECCS to depressurize the pressure vessel by steam blowdown from the steamline through a conventional safety/relief depressurization valve which discharges steam into the suppression pool within the drywell vessel. The suppression pool, therefore, is heated by the blowdown steam, and the RHR system is then used for cooling the suppression pool.
The RHR system is considered an active heat removal system since it includes selectively positionable valves and powered pumps. Passive heat removal systems are being developed to reduce or eliminate the need for such active components for improving operation and reliability of the heat removal system. For example, two passive heat removal systems are disclosed in commonly assigned applications: "Passive Heat Removal From Nuclear Reactor Containment," Ser. No. 07/432,246, filed Nov. 6, 1989, now U.S. Pat. No. 5,082,619; and "Isolation Condenser Passive Cooling Of A Nuclear Reactor Containment," Ser. No. 07/519,070, filed May 4, 1990, now U.S. Pat. No. 5,059,385. In these two applications, an isolation pool containing an isolation condenser is provided at an elevation above a gravity driven coolant supply pool which in turn is disposed at an elevation above the suppression pool which is disposed above the reactor core. The isolation condenser includes an inlet line either directly joined to the reactor pressure vessel or having an open entry for receiving steam, and an outlet line having two branches for returning condensate and non-condensable gasses to the suppression pool, or the suppression pool and the supply pool. The respective elevations of the isolation pool, the supply pool, and the suppression pool are used for passively channeling water by gravity back into the reactor pressure vessel.
As indicated above, at least some of the reactor auxiliary systems are interrelated as well as including active components. In order to improve operation and reliability of the reactor plant, passive systems such as those referenced above are being considered, as well as others.